I used to think mistletoes were largely a southern phenomenon, preferring regions with mild or even no winters. Then I was introduced to the dwarf mistletoes in the genus Arceuthobium. These odd parasites can be found growing throughout the northern hemisphere. Their affinity for conifers has landed them on the watch list of many a forester yet, despite their economic implications, the dwarf mistletoes are fascinating parasitic plants.

First and foremost, these are aggressive little plants. They vary in their host specificity. Some species can grow on a wide variety of conifer species from Abies balsamea (balsam fir), Larix laricina (American larch), to Pinus strobus (eastern white pine), whereas others are more specialized, preferring only spruces (Picea spp.). Regardless, infestations of these parasites can do some interesting things to conifer stands.

Similar to other mistletoes, the dwarfs are stem parasites. They penetrate into their hosts vascular tissues and set up shop, sucking up water and photosynthates and giving nothing in return. Because of this, large infestations can seriously drain their host trees as they themselves have reduced or even no photosynthetic capacity. Additionally, they interfere with nutrient and hormone flows throughout the branches of their host. Such disruptions can result in the formation of dense clusters of branches called "witches brooms." Some dwarf mistletoe infestations can become so intense that they effectively girdle their host tree.

In natural settings, this serves an ecological function. By weakening their hosts, dwarf mistletoes can leave room for other plant species to take root. They also keep one species from becoming too dominant. As such, mistletoe infestations can actually increase plant diversity in the long run. Dwarf mistletoe infestations only become an issue once humans get involved. They can cause serious financial issues for foresters as well as damage important or valued specimen trees. In our highly fragmented forests, their natural behavior can get in the way of human ideals.

All of this talk of damage can distract us from just how amazing some of these species really are from an organismal standpoint. For instance, the lodgepole pine dwarf mistletoe, Arceuthobium americanum, is capable of thermogenesis. Unlike the other examples of thermogenesis in the plant world, this has nothing to do with flowers. Instead, thermogenesis in A. americanum is used as a seed dispersal agent.

The dwarf mistletoes don't rely on fleshy fruits to get their seeds from one tree to another. Instead, they utilize ballistic means. As their seed pods mature, they gradually swell. Once pressure is great enough, the seed pods erupt, sending their sticky seeds flying through the canopy at speeds of up to 62 mph (100 km/h)! If lucky, the seeds will stick to the branches of a viable host or be transported there in the fur or feathers of an animal. For A. americanum, the eruption of its seed pods is triggered by heat. Using specialized metabolic pathways at the cellular level, A. americanum is able to heat its seed pods up to ~2 °C warmer than its surroundings, thus triggering its pods to explode.